[PATCH bpf-next 08/18] bpf: Support stack arguments for kfunc calls

Yonghong Song
2026-04-24T17:15:27Z

Extend the stack argument mechanism to kfunc calls, allowing kfuncs
with more than 5 parameters to receive additional arguments via the
r11-based stack arg area.

For kfuncs, the caller is a BPF program and the callee is a kernel
function. The BPF program writes outgoing args at negative r11
offsets, following the same convention as BPF-to-BPF calls:

Outgoing: r11 - 8 (arg6), ..., r11 - N*8 (last arg)

The following is an example:

int foo(int a1, int a2, int a3, int a4, int a5, int a6, int a7) {
...
kfunc1(a1, a2, a3, a4, a5, a6, a7, a8);
...
kfunc2(a1, a2, a3, a4, a5, a6, a7, a8, a9);
...
}

Caller (foo), generated by llvm
===============================
Incoming (positive offsets):
r11+8: [incoming arg 6]
r11+16: [incoming arg 7]

Outgoing for kfunc1 (negative offsets):
r11-8: [outgoing arg 6]
r11-16: [outgoing arg 7]
r11-24: [outgoing arg 8]

Outgoing for kfunc2 (negative offsets):
r11-8: [outgoing arg 6]
r11-16: [outgoing arg 7]
r11-24: [outgoing arg 8]
r11-32: [outgoing arg 9]

Later JIT will marshal outgoing arguments to the native calling
convention for kfunc1() and kfunc2().

For kfunc calls where stack args are used as constant or size
parameters, a mark_stack_arg_precision() helper is used to propagate
precision within the current frame.

There are two places where meta->release_regno needs to keep
regno for later releasing the reference. Also, 'cur_aux(env)->arg_prog = regno'
is also keeping regno for later fixup. Since stack arguments don't have a valid
register number (regno is set to -1), these three cases are rejected for now
if the argument is on the stack.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
kernel/bpf/verifier.c | 73 +++++++++++++++++++++++++++++++++----------
1 file changed, 56 insertions(+), 17 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6994536b4e04..43aeb04f488a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11559,14 +11559,12 @@ bool bpf_is_kfunc_pkt_changing(struct bpf_kfunc_call_arg_meta *meta)
}

static enum kfunc_ptr_arg_type
-get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
- struct bpf_kfunc_call_arg_meta *meta,
+get_kfunc_ptr_arg_type(struct bpf_verifier_env *env, struct bpf_func_state *caller,
+ struct bpf_reg_state *regs, struct bpf_kfunc_call_arg_meta *meta,
const struct btf_type *t, const struct btf_type *ref_t,
const char *ref_tname, const struct btf_param *args,
int arg, int nargs, argno_t argno, struct bpf_reg_state *reg)
{
- u32 regno = arg + 1;
- struct bpf_reg_state *regs = cur_regs(env);
bool arg_mem_size = false;

if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||
@@ -11575,8 +11573,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
return KF_ARG_PTR_TO_CTX;

if (arg + 1 < nargs &&
- (is_kfunc_arg_mem_size(meta->btf, &args[arg + 1], &regs[regno + 1]) ||
- is_kfunc_arg_const_mem_size(meta->btf, &args[arg + 1], &regs[regno + 1])))
+ (is_kfunc_arg_mem_size(meta->btf, &args[arg + 1], get_func_arg_reg(caller, regs, arg + 1)) ||
+ is_kfunc_arg_const_mem_size(meta->btf, &args[arg + 1], get_func_arg_reg(caller, regs, arg + 1))))
arg_mem_size = true;

/* In this function, we verify the kfunc's BTF as per the argument type,
@@ -12241,6 +12239,8 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
int insn_idx)
{
const char *func_name = meta->func_name, *ref_tname;
+ struct bpf_func_state *caller = cur_func(env);
+ struct bpf_reg_state *regs = cur_regs(env);
const struct btf *btf = meta->btf;
const struct btf_param *args;
struct btf_record *rec;
@@ -12249,9 +12249,9 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_

args = (const struct btf_param *)(meta->func_proto + 1);
nargs = btf_type_vlen(meta->func_proto);
- if (nargs > MAX_BPF_FUNC_REG_ARGS) {
+ if (nargs > MAX_BPF_FUNC_ARGS) {
verbose(env, "Function %s has %d > %d args\n", func_name, nargs,
- MAX_BPF_FUNC_REG_ARGS);
+ MAX_BPF_FUNC_ARGS);
return -EINVAL;
}
if (nargs > MAX_BPF_FUNC_REG_ARGS && !bpf_jit_supports_stack_args()) {
@@ -12260,15 +12260,20 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return -ENOTSUPP;
}

+ ret = check_outgoing_stack_args(env, caller, nargs);
+ if (ret)
+ return ret;
+
/* Check that BTF function arguments match actual types that the
* verifier sees.
*/
for (i = 0; i < nargs; i++) {
- struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[i + 1];
+ struct bpf_reg_state *reg = get_func_arg_reg(caller, regs, i);
const struct btf_type *t, *ref_t, *resolve_ret;
enum bpf_arg_type arg_type = ARG_DONTCARE;
argno_t argno = argno_from_arg(i + 1);
- u32 regno = i + 1, ref_id, type_size;
+ int regno = reg_from_argno(argno);
+ u32 ref_id, type_size;
bool is_ret_buf_sz = false;
int kf_arg_type;

@@ -12278,6 +12283,11 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
verifier_bug(env, "Only 1 prog->aux argument supported per-kfunc");
return -EFAULT;
}
+ if (regno < 0) {
+ verbose(env, "%s prog->aux cannot be a stack argument\n",
+ reg_arg_name(env, argno));
+ return -EINVAL;
+ }
meta->arg_prog = true;
cur_aux(env)->arg_prog = regno;
continue;
@@ -12304,7 +12314,10 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
reg_arg_name(env, argno));
return -EINVAL;
}
- ret = mark_chain_precision(env, regno);
+ if (regno >= 0)
+ ret = mark_chain_precision(env, regno);
+ else
+ ret = mark_stack_arg_precision(env, i);
if (ret < 0)
return ret;
meta->arg_constant.found = true;
@@ -12329,7 +12342,10 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
}

meta->r0_size = reg->var_off.value;
- ret = mark_chain_precision(env, regno);
+ if (regno >= 0)
+ ret = mark_chain_precision(env, regno);
+ else
+ ret = mark_stack_arg_precision(env, i);
if (ret)
return ret;
}
@@ -12357,14 +12373,21 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return -EFAULT;
}
meta->ref_obj_id = reg->ref_obj_id;
- if (is_kfunc_release(meta))
+ if (is_kfunc_release(meta)) {
+ if (regno < 0) {
+ verbose(env, "%s release arg cannot be a stack argument\n",
+ reg_arg_name(env, argno));
+ return -EINVAL;
+ }
meta->release_regno = regno;
+ }
}

ref_t = btf_type_skip_modifiers(btf, t->type, &ref_id);
ref_tname = btf_name_by_offset(btf, ref_t->name_off);

- kf_arg_type = get_kfunc_ptr_arg_type(env, meta, t, ref_t, ref_tname, args, i, nargs, argno, reg);
+ kf_arg_type = get_kfunc_ptr_arg_type(env, caller, regs, meta, t, ref_t, ref_tname,
+ args, i, nargs, argno, reg);
if (kf_arg_type < 0)
return kf_arg_type;

@@ -12514,6 +12537,11 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
dynptr_arg_type |= DYNPTR_TYPE_FILE;
} else if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_file_discard]) {
dynptr_arg_type |= DYNPTR_TYPE_FILE | OBJ_RELEASE;
+ if (regno < 0) {
+ verbose(env, "%s release arg cannot be a stack argument\n",
+ reg_arg_name(env, argno));
+ return -EINVAL;
+ }
meta->release_regno = regno;
} else if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_clone] &&
(dynptr_arg_type & MEM_UNINIT)) {
@@ -12668,9 +12696,9 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
break;
case KF_ARG_PTR_TO_MEM_SIZE:
{
- struct bpf_reg_state *buff_reg = &regs[regno];
+ struct bpf_reg_state *buff_reg = reg;
const struct btf_param *buff_arg = &args[i];
- struct bpf_reg_state *size_reg = &regs[regno + 1];
+ struct bpf_reg_state *size_reg = get_func_arg_reg(caller, regs, i + 1);
const struct btf_param *size_arg = &args[i + 1];
argno_t next_argno = argno_from_arg(i + 2);

@@ -13574,8 +13602,19 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
clear_all_pkt_pointers(env);

nargs = btf_type_vlen(meta.func_proto);
+ if (nargs > MAX_BPF_FUNC_REG_ARGS) {
+ struct bpf_func_state *caller = cur_func(env);
+ struct bpf_subprog_info *caller_info = &env->subprog_info[caller->subprogno];
+ u16 out_stack_arg_depth = (nargs - MAX_BPF_FUNC_REG_ARGS) * BPF_REG_SIZE;
+ u16 stack_arg_depth = caller_info->incoming_stack_arg_depth + out_stack_arg_depth;
+
+ if (stack_arg_depth > caller_info->stack_arg_depth)
+ caller_info->stack_arg_depth = stack_arg_depth;
+ invalidate_outgoing_stack_args(caller);
+ }
+
args = (const struct btf_param *)(meta.func_proto + 1);
- for (i = 0; i < nargs; i++) {
+ for (i = 0; i < min_t(int, nargs, MAX_BPF_FUNC_REG_ARGS); i++) {
u32 regno = i + 1;

t = btf_type_skip_modifiers(desc_btf, args[i].type, NULL);
--
2.52.0